Short persistence zinc sulfide blue phosphor and method of preparing same

ABSTRACT

A phosphor composition that emits blue color under cathode ray excitation and has a reduced decay time and a process for preparing same are disclosed. The phosphor composition comprises a predominant portion of silver-activated zinc sulfide and an effective amount of aluminum oxide. The process comprises forming a mixture comprising an effective amount of an aluminum source and a predominant portion of a silver-activated zinc sulfide composition and firing the mixture in an oxidizing atmosphere at a temperature of above about 1,700* F. for a time sufficient to produce said phosphor composition.

United States Patent Fisher, Jr. et al. 51 May 30, 1972 [54] SHORTPERSISTENE ZINC SULFIDE 2,474,506 6/1949 Wood ..252/301.6 s BLUEPHOSPHOR AND METHOD OF 2,600,263 6/1952 Prener.... .....252/301 .6 SPREPARING SAME 2,623,858 12/1952 Kroger.... .....252/301.6 S 3,290,25512/1966 Smith ..252/301 .6 S [72] Inventors: Philip C. Fisher, Jr.;Kenneth H. Shaner,

both of Towanda' Primary Examiner-James E. Poer 73 Assignee: SylvaniaElectric Products Inc. Amman! Examiner-l Cooper An N .1. OMall D ald R.C t] d W11 22 Filed: Apr. 29, 1971 ma ey M as e [21] Appl. No.2 138,739

ABSTRACT Related U.S. Application Data A phosphor composition that emitsblue color under cathode Commuatlon 0f Sell 815,315, P 15, 1969, rayexcitation and has a reduced decay time and a process forabandonedpreparing same are disclosed. The phosphor compositioncomprises a predominant portion of silver-activated zinc sul- E tide andan effective amount of aluminum oxide. The process comprises forming anuxture comprising an effective amount [58] Field 01 Search ..252/30l.6S of an aluminum source and a predominant portion ofa silvep activatedzinc sulfide composition and firing the mixture in an [56] ReferencesCited oxidizing atmosphere at a temperature of above about 1,700

UNITED STATES PATENTS F. for a time sufiicient to produce said phosphorcomposition.

2,421,207 5/1947 Leverenz ..252/30l.6 S 6 Claims, No Drawings SHORTPERSISTENCE ZINC SULFIDE BLUE PHOSPI-IOR AND METHOD OF PREPARING SAMEThis application is a continuation of application Ser. No. 8 16,8 l 6,filed Apr. 16, 1969, now abandoned.

BACKGROUND OF THE INVENTION This invention relates to blue-emittingcathodoluminescent phosphors. More particularly, it relates to improvedsilver-activated zinc sulfide phosphor compositions and processes forpreparing same.

Commercial picture tubes for color television include a viewing screencomprising red-emitting, green-emitting and blue-emitting phosphors. Thephosphors are generally arranged in groups of three dots, one dot foreach type of phosphor. Several factors complicate the relatively simpleprinciple thereby influencing the fidelity of the color reproduction.Among these factors is the decay characteristics of the phosphors. Whendecay characteristics of the phosphors are not matched, the viewed imageis distorted, particularly when images of moving objects are displayed.

During the past few years both the red-emitting and greenemittingphosphors have undergone considerable change. New phosphors have beendeveloped with the primary emphasis upon brightness of the phosphors.Over these years of development, the blue-emitting phosphor has beenzinc sulfide with a silver activator. This phosphor, although it has arelatively long decay time, has remained substantially unchanged. Sincebrighter and more efiicient red-emitting and green-emitting phosphorsthat have relatively short decay time have been developed, the fidelityof color television can be improved by the use of a blue-emittingphosphor with a reduced decay time, thereby reducing the blue-trailing,thus constituting an advancement in the art.

SUMMARY OF THE INVENTION In accordance with one aspect of this inventionthere is provided an improved blue-emitting phosphor composition havinga decay time significantly less than silver-activated zinc sulfidecomprising a major portion of a silver-activated zinc sulfide and anefiective amount of aluminum.

In accordance with an additional aspect of this invention, there isprovided a process for producing the improved blueemitting phosphorcomposition; the process comprises forming a relatively uniform mixtureof zinc sulfide, a silver activator and an effective amount of analuminum source and heating the mixture at a temperature of at leastabout l,700 F. for a time sufficient to convert said mixture into ablue-emitting phosphor composition and thereafter washing saidcomposition to thereby produce a blue-emitting phosphor having asignificantly reduced decay time.

For a better understanding of the present invention, together with otherand further objects, advantages and capabilities thereof, reference ismade to the following disclosure and appended claims in connection withthe above description of some of the aspects of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As was previously mentioned,the improved blue-emitting phosphors of the present invention comprise amajor or predominant portion of silver-activated zinc sulfide and aneffective amount of aluminum. Although relatively minute amounts ofaluminum, such as little as 0.01 percent by weight of the total weightof the phosphor composition, results in a phosphor composition that hasa decay time less than that of the.previously used silver-activated zincsulfide phosphors without any appreciable color shift. Compositionscontaining aluminum in amounts greater than 0.01 percent by weight andpreferably 0.2 to 1 percent by weight have been found to be particularlywell suited for the color television industry when used in conjunctionwith the newer rare earth red-emitting phosphors.

In the manufacture of the blue-emitting zinc sulfide phosphorsheretofore used, alkaline earth metal or alkali metal salt fluxes suchas the chlorides, phosphates and nitrates, were generally employed as amixture with the zinc sulfide and the silver activator. During theheating or firing of the zinc sulfide, to convert it into a phosphor,the fluxes served to produce a phosphor having the desired particlesize. Generally, the individual fluxes are used in amounts of less thanabout 5 percent by weight of the zinc sulfide raw materials used to makethe phosphor composition. In the practice of the present invention, theforegoing fluxes, such as sodium chloride, barium chloride, magnesiumchloride, can be employed in the amounts heretofore indicated. In mostinstances the anion portion forms the volatile chlorine gas and isevolved during the heating cycle. The cation portion of the flux cancombine with oxygen to form the metal oxide and remain in the phosphoras an oxide or be incorporated into the matrix as the metal form of thecorresponding cation. and are generally in amounts of from about 0.01percent to about 0.2 percent by weight of the phosphor. The alkali metalcation is generally from about 0.01 percent to about 0.05 percent byweight of the phosphor, and the alkaline earth metal cation is found atfrom about 0.01 percent to about 0.15 percent by weight of the phosphor.

As previously mentioned, silver is used as the activator for theblue-emitting zinc sulfide phosphor. In most instances, a silver salt isemployed that has a fugitive or volatile anion, that is an anion thatwill form a volatile component upon the thermal decomposition of thesalt. Such salts must decompose in the foregoing manner at temperaturesof below about l,700 F. Silver, in its metallic form, is then combinedin the matrix of the host zinc sulfide phosphor to yield ablue-emitting, cathodoluminescent phosphor.

In the practice of this invention, an aluminum source is employed thatthermally decomposes to form a volatile or fugitive component derivedfrom the anion portion of the aluminum source and the aluminum portionof the source forms either the metallic form of aluminum or aluminumoxide. Aluminum trichloride hexahydrate is a preferred aluminum source.Upon heating to a temperature of above about 1,200 F chlorine gas isevolved and the aluminum can form aluminum oxide under oxidizingconditions. Other aluminum sources can be used such as aluminum nitrate,sulfate and the like. Generally, aluminum salts of the mineral acids arepreferred with aluminum chloride hexahydrate being especially preferred.

It is believed to be surprising that aluminum gives the desired effectbecause many aluminum containing materials, including aluminum oxide,are activators for various phosphors, and therefore, would normally beexpected to cause a significant color shift in the blue-emittingphosphor. The phosphor composition of this invention, however, exhibitsessentially the same characteristics with the exception of the decreaseddecay time, as silver-activated zinc sulfide phosphor compositionswithout the aluminum.

The change in decay time is related to the amount of aluminum employedin the raw material used to make the phosphor composition at levels ofaluminum below about 6 percent by weight. More than 6 percent does notappreciably reduce the decay time below that achieved at lower levelssuch as 4 to 5 percent, and can result in a change of at least some ofthe other characteristics of the blue-emitting phosphor. While thesehigher levels of aluminum can be used, in most instances these higherlevels have no appreciable beneficial effect over the preferred levelshence are not normally used. As the raw materials are heated, thenwashed to produce the phosphor composition, the amount of aluminum inthe composition will vary within the range of from about 0.01 percent toabout 1.0 percent by weight of the phosphor composition.

After the mixture of zinc sulfide, a silver salt containing a fugitiveanion and an aluminum source is formed, the mixture is heated to atleast about 1,700 F. for a sufficient period of time to convert thematerial into the desired blue-emitting phosphor. The period of heatingis known in the art of producing zinc sulfide phosphors and it is notdetailed herein. Since both the activator, the fluxes and the aluminumsources are generally comprised of materials having a volatile orfugitive cation, the time period of heating is essentially unchangedfrom that used in producing the prior art blue-emitting zinc sulfidephosphors.

After the heating step, the phosphor is washed in deionized water toremove any residual and unreacted fluxes, persistance modifiers and thelike.

After being washed, the solid silver-activated phosphors containing theeffective amounts of aluminum, are removed by conventional solid-liquidseparation means, preferably by filtration. After the solid, phosphorcompositions are removed from the mother liquor or the liquid phase, thesolids are dried at about 1 10 C. and are then ready for commercial use.

To more fully illustrate some of the aspects of this invention,'thefollowing detailed examples are presented. All parts, percentages andproportions are by weight unless otherwise indicated.

EXAMPLE I.

A relatively uniform mixture of about 100 parts of zinc sulfide, about 3parts of barium chloride dihydrate, about 3 parts of aluminum chloridehexahydrate and about 0.05 parts of silver nitrate are prepared bymixing the foregoing ingredients in a conventional ribbon blender forabout 45 minutes.

After the uniform mixture is prepared it is fired at a temperature ofabout l,750 F. for about 2 hours. After the fired material is'cooled itis washed with deionized water by slurrying the fired material in about8 parts of water. The slurry is then filtered and the solids are removedand dried at about 1 10 C. until essentially no moisture is detected inthe material.

Samples of material that pass through the openings in a 325 mesh ([15.Standard Sieve) indicate equivalent brightness to samples prepared inthe same manner without aluminum chloride hexahydrate. The decay time ofthe phosphor is found to be about one-half of that of the phosphorprepared without the aluminum chloride.

The elemental analysis of a sample of the material indicates thefollowing amounts of components in percentages by weight:

Zn 66.8% Ag 0.027% Ba 0.1% Al 0.43% M 0.05% S 31.8% CI 0.014%

The above analysis indicates that essentially all of the chloride andnitrate is evolved during the heating operation but that significantamounts of the metals are retained in the phosphor composition.

EXAMPLE ll Other phosphors are parepared in a similar manner to thatprepared in Example I except that different fluxes are used. Theingredients used are shown in Table 1 following:

The decay time of the samples as measured by the Rolling RasterTechnique which is a ratio of the decay time of a test phosphor to thatof a standard phosphor is given in Table ll 10 following:

fide phosphors as normally used in present color television screens andtested by the same technique range from about 1.75 for the shortestdecay time to about 2.43.

It can be seen, therefore, that the phosphors of the present inventionhave an appreciable shorter decay time than do the phosphors of theprior art.

While there has been shown and described what are, at present,considered the preferred embodiments of the invention, it will beobvious to those skilled in the art that various changes andmodifications may be made therein without departing from the scope ofthe invention as defined by the appended claims.

We claim:

1. A blue-emitting cathodoluminescent phosphor composition having areduced decay time consisting essentially of silver-activated zincsulfide host, from about 0.01 percent to about 1 percent by weight ofaluminum, as aluminum oxide, from about 0.01 percent to about 0.05percent by weight of an alkali metal and from about 0.01 percent toabout 0.15 percent by weight of an alkaline earth metal.

2. A composition according to claim 1 wherein said alkaline earth metalis barium.

3. A composition according to claim 2 wherein said alkali metal issodium.

4. A process for preparing an aluminum oxide containing silver activatedzinc sulfide blue-emitting cathodoluminescent phosphor compositionhaving a reduced decay time comprismg:

a. forming a relatively uniform mixture consisting essentially of zincsulfide, a silver source selected from silver salts having adecomposition temperature below l,700 F., from about 0.1 percent toabout 6 percent by weight of a thermally decomposable aluminum source,greater than about 0.01 percent to less than about 5 percent by weightof an alkali metal flux and greater than about 0.01 percent and lessthan about 5 percent by weight of an alkaline earth metal flux;

b. heating said mixture to a temperature of at least about l,700 F. inan oxidizing atmosphere for a time sufficient to produce a blue-emittingphosphor composition, and

c. washing said composition with deionized water.

5. A process according to claim 4 wherein said alkali metal is sodium. Il

6. A process according to claim 5 wherein said alkaline 7Q earth metalis selected from the group consisting of barium,

magnesium and mixtures thereof.

2. A composition according to claim 1 wherein said alkaline earth metalis barium.
 3. A composition according to claim 2 wherein said alkalimetal is sodium.
 4. A process for preparing an aluminum oxide containingsilver activated zinc sulfide blue-emitting cathodoluminescent phosphorcomposition having a reduced decay time comprising: a. forming arelatively uniform mixture consisting essentially of zinc sulfide, asilver source selected from silver salts having a decompositiontemperature below 1,700* F., from about 0.1 percent to about 6 percentby weight of a thermally decomposable aluminum source, greater thanabout 0.01 percent to less than about 5 percent by weight of an alkalimetal flux and greater than about 0.01 percent and less than about 5percent by weight of an alkaline earth metal flux; b. heating saidmixture to a temperature of at least about 1, 700* F. in an oxidizingatmosphere for a time sufficient to produce a blue-emitting phosphorcomposition, and c. washing said composition with deionized water.
 5. Aprocess according to claim 4 wherein said alkali metal is sodium.
 6. Aprocess according to claim 5 wherein said alkaline earth metal isselected from the group consisting of barium, magnesium and mixturesthereof.